1. Field of the Invention
The present invention relates to a light emitting device with a light emitting element that emits fluorescent light or phosphorescent light upon application of electric field to a pair of electrodes of the element which sandwich a organic compound-containing layer (hereinafter, an electroluminescence film), and to a method of manufacturing the light emitting device. In the present invention, the term light emitting device includes an image display device using a light emitting element, a light emitting device and a light source (including illuminating device). Also, the following modules are included in the definition of the light emitting device: a module obtained by attaching to a light emitting device a connector such as an FPC (flexible printed circuit; terminal portion), a TAB (tape automated bonding) tape, or a TCP (tape carrier package); a module in which a printed wiring board is provided at an end of the TAB tape or the TCP; and a module in which an IC (integrated circuit) is directly mounted to a light emitting element by the COG (chip on glass) system.
2. Description of the Related Art
Light emitting elements, which employ materials as light emitting member and are characterized by their thinness and light-weight, fast response, and direct current low voltage driving, are expected to develop into next-generation flat panel displays. Among display devices, ones having light emitting elements arranged to form a matrix shape are considered to be particularly superior to the conventional liquid crystal display devices for their wide viewing angle and excellent visibility.
It is said that light emitting elements emit light through the following mechanism: a voltage is applied between a pair of electrodes that sandwich a light emitting layer comprising an organic material, electrons injected from the cathode and holes injected from the anode are re-combined at the luminescent center of the electroluminescence film to form molecular excitons, and the molecular excitons return to the base state while releasing energy to cause the light emitting element to emit light. Known as excitation states are singlet excitation and triplet excitation, and it is considered that luminescence can be conducted through either one of those excitation states.
Such light emitting devices having can be arranged to form a matrix can employ passive matrix driving (simple matrix light emitting devices), active matrix driving (active matrix light emitting devices), or other driving methods. However, if the pixel density is increased, active matrix light emitting devices in which each pixel (or each dot) has a switch are considered as advantageous because they can be driven with low voltage.
As an active matrix light emitting device, it is generally that a thin film transistor (hereinafter, TFT) is formed on an insulating film, then, a light emitting element that is connected electrically to TFT is formed via an interlayer insulating film that is formed on the TFT. In addition, the light emitting element is composed of an anode, a cathode, an electroluminescence film, and a cathode.
The mainstream structure of conventional active matrix light emitting device is a bottom emission structure that light generated in an electroluminescence film in a light emitting element composed of an anode connected electrically to TFT, an electroluminescence film, and a cathode in this order (the normally laminated form) is emitted through an anode to a TFT side (that is, a substrate side). Thus, the material for an anode in this case is a conductive material that transmits visible light and has a large ionization potential (which has the same value as that of a work function in case of metal) is used, typically, indium tin oxide, indium zinc oxide.
However, there is a problem in that an opening ratio is restricted depending on a position of a TFT, wirings, or the like when resolution is to be improved. In recent years, the top emission structure that emits light through a cathode side (an electrode side that is formed on an electroluminescence film and positioned opposite to a substrate) is brought to attention, for example, patent reference 1: Kokai 2001-43980.
However, in manufacturing a light emitting device of a top emission structure, in a case of using a transparent conductive film made of ITO or the like, which is appropriate as an anode material for a light emitting element when forming the normally laminated ones as described above, problems arise in that visible light is allowed to pass therethrough, which leads to light leak on a substrate side. Therefore, according to such a merit that an ionization potential is large and conductivity is achieved, when the transparent conductive film is used as an anode of the top emission element in the normally laminated form, there is a need to shield the light by forming a material into a film, through which the light cannot pass, below the transparent conductive film or the like, resulting in a complicated process.
Also, when using a metal film as the anode of the light emitting element in the normally laminated form, which is of the top emission structure, the light can be shielded. Considering materials having ionization potentials as high as ITO exhibits, however, noble metals such as gold and platinum only suffice for the above requirement, which causes problems in terms of costs. Also, there arises a problem in that the metal exhibits poor adhesion to materials for forming an electroluminescence film.
On the other hand, provided that an electrode electrically connected with a TFT serves as a cathode and the light emitting element (in the reversely laminated form) formed by sequentially laminating the electroluminescence film and the anode on the cathode is used to attain the top emission structure, the anode may be made of ITO. Thus, it becomes easy to take out the light from the top surface. In this case, however, the metal having the low ionization potential is used as the cathode, which causes a problem in that the adhesion to the materials for forming the electroluminescence film is insufficient to thereby make it difficult to inject electrons thereinto.
That is to say, regarding the light emitting element having the top emission structure in either normally or reversely laminated ones, the above-mentioned difficulty is involved due to the limitations of ionization potential of the electrode.